The coating layer of a cutting tool is important in increasing the abrasion resistance, reducing the friction of the tool and improving the tool life. When the surface roughness of a coating layer on the surface of the cutting tool is high, cutting resistance increases. This causes the cutting tool to easily adhere to a workpiece and results in the separation of the coating layer therefrom. Thus, the performance of the cutting tool is deteriorated and the tool life is shortened. To prevent this problem, it is necessary to improve the surface roughness of a coating layer as well as the bonding force between the coating layer and the substrate of the cutting tool.
The substrate of a cemented carbide cutting tool is made by molding tungsten carbide (WC) powders and cobalt (Co) powders into a desired shape, heating and sintering the same at a high temperature. Since the surface of the substrate is directly exposed to temperature changes during the sintering step, tungsten carbide and cobalt are formed on the surface in a non-uniform structure and severe unevenness also results. In contrast, the inner side of the substrate has a uniform and fine structure. When a coating layer is formed on such a substrate surface of the cemented carbide cutting tool, the non-uniform structure of the cemented carbide and cobalt on the substrate surface interrupts the uniform formation of the coating layer. Further, the bonding force between the coating layer and the substrate becomes weak so that the coating layer is easily separated. Thus, notwithstanding the coating layer formed on the substrate of the cemented carbide cutting tool, the basic objective of the coating (i.e., improving tool performance and extending tool life) cannot be achieved. Also, when a coating layer is formed by a conventional coating technique such as arc ion plating, macro droplets are formed on the surface of the coating layer such that the surface roughness of the coating layer is more degraded during the coating process.
To improve these problems, a “post-treating process” may be applied after the coating process to improve the surface roughness of the coating layer. However, this process does not effectively improve the surface roughness of the coated cemented carbide cutting tool. Additionally, it cannot essentially improve the bonding force between the coating layer and the substrate since the bonding force of the coating layer to the substrate is not improved by post-treatment of the coating layer.
Alternatively, blasting may be applied to the substrate as a “pre-treating process,” which treats the substrate surface of a cemented carbide cuffing tool before the coating process. By injecting hard particles on the surface of the cutting tool substrate at a high speed, the blasting removes a part of the surface and dusts thereon and polishes the surface. The hard particles, such as alumina, have a size of tens of microns. However, even with the blasting, the non-uniform structure and severe irregularities are not sufficiently removed. Further, it is difficult to sufficiently improve the surface roughness of the coating layer and the bonding force between the coating layer and the substrate.